While common rail is standard on most European auto and light-truck engines, Volkswagen remains committed to electronic unit injection. Figure 6-4 illustrates the Bosch unit injector of the type currently used by VW and its SEAT and Skoda affiliates. A solenoid-actuated valve controls injector timing and duration, which of course, must occur during periods when the UI is pressurized by the camshaft. Caterpillar and Detroit Diesel developed their own versions of electronic unit injectors (EUIs) described below.

In summary, computer-controlled c-r and unit-injector (UI) systems came about because of the need to reduce exhaust emissions by providing more precise control over fueling. Secondary effects are higher power outputs, more flexible torque curves, and better fuel economy.